Manufacturing method of aluminum structure and aluminum structure

ABSTRACT

There is provided a manufacturing method of an aluminum structure, including a conductive treatment process of forming an electrically conductive layer made of aluminum on a surface of a resin molded body and a plating process of plating the resin molded body subjected to the conductive treatment process with aluminum in a molten salt bath. Even with a porous resin molded body having a three-dimensional network structure, the method allows the surface of the porous resin molded body to be plated with aluminum, thus forming a high-purity aluminum structure having a uniform thick film. Porous aluminum having a large area is also provided.

RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/JP2011/058782, which claims the benefit of priority from JapanesePatent Application No. 2010-098335, filed on Apr. 22, 2010each of whichis hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a method for forming an aluminumstructure on a resin surface by aluminum plating and, more particularly,to an aluminum structure that can be suitably used as a porous metalbody in applications, such as various filters and battery electrodes,and a manufacturing method of the aluminum structure.

BACKGROUND ART

Porous metal bodies having a three-dimensional network structure havebeen used in a wide range of applications, such as various filters,catalyst supports, and battery electrodes. For example, Celmet(manufactured by Sumitomo Electric Industries, Ltd., registeredtrademark) made of nickel has been used as an electrode material forbatteries, such as nickel-hydrogen batteries and nickel-cadmiumbatteries. Celmet is a porous metal body having continuous pores andcharacteristically has a higher porosity (90% or more) than other porousbodies, such as metal non-woven fabrics. Celmet can be manufactured byforming a nickel layer on a surface of the skeleton of a porous resinhaving continuous pores, such as urethane foam, decomposing the resinexpansion molded body by heat treatment, and reducing the nickel. Thenickel layer can be formed by performing a conductive treatment ofapplying a carbon powder to the surface of the skeleton of the resinexpansion molded body and then depositing nickel by electrodeposition.

Aluminum has excellent characteristics, such as conductive property,corrosion resistance property, and lightweight. For use in batteries,for example, aluminum foil to which an active material, such as lithiumcobalt oxide, is applied has been used as a positive electrode oflithium-ion batteries. In order to increase the capacity of a positiveelectrode, an aluminum body can be processed into a porous body having alarge surface area, and the inside of the aluminum body can be filledwith an active material. This allows the active material to be utilizedeven in an electrode having a large thickness and improves the activematerial availability ratio per unit area.

As a manufacturing method of porous aluminum, Patent Literature 1describes a method for subjecting a plastic substrate having an innercontinuous space and a three-dimensional network to an aluminum vapordeposition process by an arc ion plating method to form a metallicaluminum layer having a thickness in the range of 2 to 20 μm. PatentLiterature 2 describes a method for forming a porous metal body,including forming a film made of a metal (such as copper) on theskeleton of a resin expansion molded body having a three-dimensionalnetwork structure, the metal having an ability to form an eutectic alloyat a temperature of the melting point of aluminum or less, applying analuminum paste to the film, and performing heat treatment in anon-oxidizing atmosphere at a temperature of 550° C. or more and 750° C.or less to evaporate the organic constituent (resin foam) and sinter thealuminum powder.

Since aluminum has high chemical affinity to oxygen and a lower electricpotential than hydrogen, the electrodeposition in a plating bathcontaining an aqueous solution is difficult to perform in aluminumplating. Aluminum electrodeposition has been studied in a plating bathcontaining a non-aqueous solution, in particular a plating bathcontaining an organic solvent. For example, as a technique for plating ametal surface with aluminum, Patent Literature 3 discloses an aluminumelectrodeposition method characterized in that a low meltingcomposition, which is a blend melt of an onium halide and an aluminumhalogenide, is used in a plating bath, and aluminum is deposited on acathode while the water content of the plating bath is maintained at 2%by weight or less.

Citation List Patent Literature

PTL 1: Japanese Patent No. 3413662

PTL 2: Japanese Unexamined Patent Application Publication No. 8-170126

PTL 3: Japanese Patent No. 3202072

SUMMARY OF INVENTION Technical Problem

In accordance with the method described in Patent Literature 1, porousaluminum having a thickness in the range of 2 to 20 μm can bemanufactured. However, it is difficult to produce a large area by thegas phase method and, depending on the thickness or porosity of asubstrate; it is difficult to form a layer having a uniform interior.There are additional problems of a low rate of formation of the aluminumlayer and high manufacturing costs because of expensive installation.Furthermore, the formation of a thick film may cause cracking in thefilm or falling of aluminum. In accordance with the method described inPatent Literature 2, unfortunately, a layer that forms an eutectic alloywith aluminum is formed instead of a high-purity aluminum layer.Although an aluminum electrodeposition method is known, plating of onlya metal surface is possible, and there is no known method forelectrodeposition on a resin surface, in particular electrodeposition onthe surface of a porous resin molded body having a three-dimensionalnetwork structure. This is probably influenced by the dissolution of aporous resin in a plating bath and other problems.

Accordingly, it is an object of the present invention to provide amethod for forming a high-purity aluminum structure, includingperforming aluminum plating on the surface of a resin molded body, inparticular even a porous resin molded body having a three-dimensionalnetwork structure, to form a uniform thick film, and a manufacturingmethod of porous aluminum having a large area.

Solution to Problem

In order to solve the problems described above, the present inventorshave arrived at a method for aluminum electrodeposition of a surface ofa resin molded body made of polyurethane, melamine, or the like. Thepresent invention provides a manufacturing method of an aluminumstructure, including a conductive treatment process of forming anelectrically conductive layer made of aluminum on a surface of a resinmolded body and a plating process of plating the resin molded bodysubjected to the conductive treatment process with aluminum in a moltensalt bath (the first invention of the present application). As describedabove, although aluminum plating has been performed on metal surfaces,electrodeposition of resin molded body surfaces has not been considered.The present invention is characterized in that making a resin moldedbody surface be electrically conductive (conductive treatment) was foundto make it possible to perform aluminum plating in a molten salt bath.Furthermore, conductive treatment performed by forming an electricallyconductive layer made of aluminum can produce an aluminum structuresubstantially free of metals other than aluminum.

Since aluminum can easily react with oxygen, a thin oxide film tends tobe formed on a surface of an electrically conductive layer made ofaluminum. The oxide film reduces plating adhesion and therefore resultsin poor plating. Thus, it is preferable to provide an anode electrolysisprocess of performing electrolysis treatment using the electricallyconductive layer as an anode between the conductive treatment processand the plating process (the second invention of the presentapplication). The anode electrolysis treatment can melt and remove anoxide film formed on the surface of the electrically conductive layer inthe conductive treatment process, allowing satisfactory aluminum platingin a molten salt.

Preferably, the resin molded body subjected to the conductive treatmentprocess is transported between the conductive treatment process and theplating process without being exposed to an oxidizing atmosphere (thethird invention of the present application). This allows satisfactoryaluminum plating in a molten salt without oxidation of the electricallyconductive layer.

The conductive treatment process may be a process of depositing aluminumon the surface of the resin molded body by a gas phase method (thefourth invention of the present application). The conductive treatmentprocess may also be a process of dipping the resin molded body in acoating material containing aluminum to deposit aluminum on the resinmolded body (the fifth invention of the present application). Both ofthese methods allow the manufacture of a structure substantiallycomposed of aluminum as a metal without the contamination of metalsother than aluminum.

Such a process allows the formation of a uniform thick aluminum layer ona surface of a complicated skeleton structure, in particular a porousresin article having a three-dimensional network structure (the sixthinvention of the present application). The resin molded body ispreferably made of urethane or melamine, with which a porous resinarticle having a high porosity can be manufactured (the seventhinvention of the present application).

An aluminum structure that includes a resin molded body having a metallayer on a surface thereof is manufactured through these processes (theeleventh invention of the present application). Depending on theapplication, such as a filter or a catalyst support, the aluminumstructure may be directly used as a resin-metal composite. In order touse a metal structure without resin owing to constraints resulting fromthe usage environment, the resin may be removed (the eighth invention ofthe present application).

An aluminum structure manufactured by one of the methods described aboveincludes an aluminum layer having a thickness in the range of 1 to 100μm as a metal layer, wherein the whole metal layer without the resin hasan aluminum purity of 99.0% or more and a carbon content of 1.0% or lessand contains inevitable impurities as the balance (the tenth inventionof the present application). The carbon content is measured by aninfrared absorption method after combustion in a high-frequencyinduction furnace in accordance with Japan Industrial Standard G1211.The aluminum purity is measured with an inductively-coupled plasmaemission spectrometer after the aluminum structure has been dissolved innitromuriatic acid.

When a porous resin having a three-dimensional network structure is usedas the resin, the aluminum structure thus manufactured includes analuminum layer having a tubular skeleton structure and forming a porousbody having generally contiguous pores (the twelfth invention of thepresent application).

An aluminum structure can also be manufactured in which the skeletonstructure has almost triangular sections, and the aluminum layer has alarger thickness at the vertexes of each of the triangular sections thanat the middle of each side of the triangular sections (the thirteenthinvention of the present application).

When a urethane foam or a melamine foam having a three-dimensionalnetwork structure is used as the porous resin molded body, the skeletonof the network structure generally has triangular sections. The term“triangular”, as used herein, has no stringent definition and refers toa shape having approximately three vertexes and three curved lines asthe sides. Thus, the shape of the aluminum structure formed by platingalso has an almost triangular skeleton. As an example of the conductivetreatment method, the deposition of aluminum by a gas phase method willbe described below. An electrically conductive layer having a relativelyuniform thickness can be formed by a gas phase method. The conductivityof the electrically conductive layer is substantially constant at allpositions on each of the triangular sections. In aluminum plating undersuch conditions, an electric field is concentrated at the corners (thevertexes of a triangular section), resulting in a greater thickness atthe vertexes than at the middle of each side of the triangular section.Thus, the shape described above can be achieved. Such a shape canadvantageously increase the strength of the tubular skeleton structureand improve the retention capacity of an active material in batteryelectrodes and other applications.

Advantageous Effects of Invention

The present invention can provide a method for performing aluminumplating on the surface of a resin molded body, in particular the surfaceof a porous resin molded body having a three-dimensional networkstructure, and forming a high-purity, large-area aluminum structurehaving a substantially uniform and large thickness. The presentinvention can also provide an aluminum structure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart of a process of manufacturing an aluminumstructure according to the present invention.

FIG. 2 shows schematic cross-sectional views of a process ofmanufacturing an aluminum structure according to the present invention.

FIG. 3 is an enlarged photograph of a surface of the structure of aurethane foam as an example of a porous resin molded body.

FIG. 4 is a schematic view of a cross-section of the skeleton of porousaluminum.

FIG. 5 is an explanatory view of a continuous aluminum plating processutilizing molten salt plating.

FIG. 6 is a schematic cross-sectional view of a structure in whichporous aluminum is applied to a molten salt battery.

FIG. 7 is a schematic cross-sectional view of a structure in whichporous aluminum is applied to an electrical double layer capacitor.

FIG. 8 is a scanning electron microscope (SEM) photograph of across-section of porous aluminum.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below in which arepresentative example is a process of manufacturing porous aluminum.Throughout the reference figures, like numerals designate like parts.The dimensions in the figures are not necessarily consistent with theirdescriptions. The present invention is defined by the appended claimsrather than by these embodiments. All modifications that fall within thescope of the claims and the equivalents thereof are intended to beembraced by the claims.

(Process of Manufacturing Aluminum Structure)

FIG. 1 is a flow chart of a process of manufacturing an aluminumstructure according to the present invention. FIG. 2 shows schematicviews of the formation of an aluminum structure using a resin moldedbody as a core material in accordance with the flow chart. The generalflow of the manufacturing process will be described below with referenceto these figures. First, the preparation of a base resin molded body 101is performed. FIG. 2( a) is an enlarged schematic view of a portion of across-section of a resin, which is the magnification of a surface of aresin expansion molded body having continuous pores serving as anexample of a base resin molded body. Pores are formed in the skeleton ofa resin expansion molded body 1. The conductive treatment of the surfaceof the resin molded body 102 is then performed. As illustrated in FIG.2( b), through this process, a thin electrically conductive layer 2 madeof aluminum is formed on the surface of the resin molded body 1.Aluminum plating in a molten salt 103 is then performed to form analuminum plated layer 3 on the surface of the electrically conductivelayer of the resin molded body (FIG. 2( c)). Thus, an aluminum structureis manufactured in which the aluminum plated layer 3 is formed on asurface of a base resin molded body serving as the base material.Removal of the base material formed by a base resin molded body 104 maybe further performed. The resin expansion molded body 1 can beevaporated by decomposition to form an aluminum structure (porous body)containing only the metal layer (FIG. 2( d)). These processes will bedescribed below process by process.

(Preparation of Porous Resin Molded Body)

A porous resin molded body having a three-dimensional network structureand continuous pores is prepared. The material of the porous resinmolded body may be any resin. The material may be exemplified by a resinexpansion molded body made of polyurethane, melamine, polypropylene, orpolyethylene. The resin expansion molded body may be a resin molded bodyhaving any shape provided that the resin molded body has contiguouspores (continuous pores). For example, a nonwoven fabric containingtangled fibrous resin may be used in place of the resin expansion moldedbody. Preferably, the resin expansion molded body has a porosity in therange of 80% to 98% and a pore size in the range of 50 to 500 μm.Urethane foams and melamine foams have a high porosity, continuouspores, and an excellent pyrolysis property and are therefore suitablefor the resin expansion molded body. Urethane foams are preferred interms of the uniformity of pores and availability. Urethane foams arepreferred because of their small pore size.

Porous resin molded bodies often contain residue materials, such as afoaming agent and an unreacted monomer in the manufacture of the foam,and are therefore preferably subjected to washing treatment before thesubsequent processes. As an example of the porous resin molded body,FIG. 3 illustrates a urethane foam subjected to a washing treatment as apreliminary treatment. The resin molded body has a three-dimensionalnetwork skeleton, which includes generally contiguous pores. Theskeleton of the urethane foam has an almost triangular sectionperpendicular to the lateral direction. The porosity is defined by thefollowing equation:

Porosity=(1−(the weight of porous body [g]/(the volume of porous body[cm³]×material density)))×100 [%]

The pore size is determined by magnifying a surface of the resin moldedbody in a photomicrograph or the like, counting the number of cells perinch (25.4 mm), and calculating the average pore size by the followingequation: average pore size=25.4 mm/the number of cells.

(Conductive Treatment of Resin Molded Body Surface: Gas Phase Method)

An electrically conductive layer made of aluminum is formed on thesurface of a resin expansion molded body. The electrically conductivelayer may be formed by any method, for example, a gas phase method, suchas vapor deposition, sputtering, or plasma chemical vapor deposition(CVD), or application of an aluminum paint. A vapor deposition method ispreferred because a thin film can be uniformly formed. Preferably, theelectrically conductive layer has a thickness in the range of 0.05 to 1μm, preferably 0.1 to 0.5 μm. When the electrically conductive layer hasa thickness of less than 0.01 μm, conductive treatment is insufficient,and electrolytic plating cannot be properly performed in the nextprocess. A thickness of more than 1 μm results in an increase in thecost of the conductive treatment process.

(Conductive Treatment of Resin Molded Body Surface: Coating Material)

The conductive treatment may be performed by dipping a resin expansionmolded body in a coating material containing aluminum. The aluminumcomponent in the coating material is deposited on the surface of theresin expansion molded body to form an electrically conductive layermade of aluminum, producing an electrically conductive state that allowsplating in a molten salt. The coating material containing aluminum maybe a liquid containing aluminum fine particles having a particlediameter in the range of 10 nm to 1 μm dispersed in water or an organicsolvent. The resin foam can be dipped in the coating material and heatedto evaporate the solvent to form the electrically conductive layer.

(Pretreatment for Plating: Anode Electrolysis)

Aluminum is plated by molten salt plating on the electrically conductivelayer formed by the process described above to form an aluminum platedlayer. The presence of an oxide film on the surface of the electricallyconductive layer may result in a poor adhesive property of aluminum inthe next plating process, resulting in the deposition of island-shapedaluminum or variations in the thickness of the aluminum plated layer.Thus, an anode electrolysis treatment is preferably performed before theplating process to dissolve and remove an oxide film (aluminum oxidelayer) formed on the electrically conductive layer (aluminum layer).More specifically, while a resin molded body subjected to conductivetreatment and a counter electrode, such as an aluminum sheet, is dippedin a molten salt, a direct current is applied between the resin moldedbody subjected to conductive treatment (an electrically conductivelayer) functioning as an anode and the counter electrode functioning asa cathode. The molten salt may be the same as or different from themolten salt used in the next molten salt plating process.

(Pretreatment for Plating: Non-oxidizing atmosphere)

In accordance with another method for preventing the oxidation of anelectrically conductive layer (aluminum layer), after the electricallyconductive layer has been formed, a resin molded body having theelectrically conductive layer (a resin molded body subjected toconductive treatment) is transported to the next plating process withoutbeing exposed to an oxidizing atmosphere. For example, a vapordeposition apparatus and a molten salt plating apparatus are placed inan argon atmosphere. After a conductive treatment process utilizingvapor deposition is performed in an argon atmosphere, the sample istransported in an argon atmosphere to the next process, in which moltensalt plating is performed. Thus, the surface of the electricallyconductive layer formed in the conductive treatment process can beplated without oxidation.

(Formation of Aluminum Layer: Molten Salt Plating)

The aluminum plated layer 3 is then formed on the surface of the resinmolded body by electrolytic plating in a molten salt. A direct currentis applied between a cathode of the resin molded body having a surfacesubjected to conductive treatment and an anode of a 99.99% aluminumplate in a molten salt. The aluminum plated layer has a thickness in therange of 1 to 100 μm, preferably 5 to 20 μm. In contrast to the anodeelectrolysis treatment, a direct current is applied between a cathode ofthe resin molded body subjected to conductive treatment and an anode ofthe counter electrode in a molten salt. The molten salt may be anorganic molten salt that is an eutectic salt of an organic halide and analuminum halogenide or an inorganic molten salt that is an eutectic saltof an alkaline metal halide and an aluminum halogenide. Use of a bath ofan organic molten salt that can melt at a relatively low temperature ispreferred because it allows plating without the decomposition of thebase material, a resin molded body. The organic halide may be animidazolium salt or a pyridinium salt. Among others,1-ethyl-3-methylimidazolium chloride (EMIC) and butylpyridinium chloride(BPC) are preferred. The imidazolium salt is preferably a salt thatcontains an imidazolium cation having alkyl groups at 1,3-position. Inparticular, aluminum chloride and 1-ethyl-3-methylimidazolium chloride(AlCl₃-EMIC) molten salts are most preferred because of their highstability and resistance to decomposition.

The contamination of a molten salt by water or oxygen causes adeterioration of the molten salt. Thus, plating is preferably performedin an atmosphere of an inert gas, such as nitrogen or argon, in a sealedenvironment. When an EMIC bath is used as the organic molten salt bath,the temperature of the plating bath ranges from 10° C. to 60° C.,preferably 25° C. to 45° C.

FIG. 5 is a schematic view of an apparatus for continuously performing ametal plating treatment of a strip of resin. A strip of resin 22 havinga surface subjected to conductive treatment is transferred from the leftto the right in the figure. A first plating bath 21 a includes acylindrical electrode 24, a positive electrode 25 disposed on the innerwall of a container, and a plating bath 23. The strip of resin 22 passesthrough the plating bath 23 along the cylindrical electrode 24. Thus, auniform electric current can easily flow through the entire resin,achieving uniform plating. A plating bath 21 b for performing thickuniform plating is composed of a plurality of baths so that plating canbe performed multiple times. The strip of resin 22 having a thin metalbath on a surface thereof is transferred by electrode rollers 26, whichfunction as feed rollers and power feeding cathodes on the outside ofcontainer, through a plating bath 28 to perform plating. The pluralityof baths include positive electrodes 27 facing both faces of the resinvia the plating bath 28, allowing more uniform plating on both faces ofthe resin.

An aluminum structure (porous aluminum) having a resin molded body asthe core of its skeleton is manufactured through these processes.Depending on the application, such as a filter or a catalyst support,the aluminum structure may be directly used as a resin-metal composite.In order to use a metal structure without resin because of constraintsresulting from the usage environment, the resin may be removed. Theresin may be removed by decomposition (dissolution) with an organicsolvent, a molten salt, or supercritical water, decomposition byheating, or any other method. Decomposition by heating at hightemperature is convenient but causes the oxidation of aluminum. Unlikenickel, once oxidized, aluminum is difficult to reduce. Thus, for use inan electrode material for batteries, aluminum cannot be used because itsconductive property is lost by oxidation. In order to prevent theoxidation of aluminum, therefore, a method for removing a resin bydecomposition by heating in a molten salt as described below ispreferably used.

(Removal of Resin: Decomposition by Heating in Molten Salt)

Decomposition by heating in a molten salt is performed in the followingmanner. A resin expansion molded body having an aluminum plated layer ona surface thereof is dipped in a molten salt. The resin expansion moldedbody is decomposed by heating while a negative potential is applied tothe aluminum layer. The application of the negative potential whiledipping the resin expansion molded body in the molten salt can preventthe oxidation of aluminum. Heating under such conditions allows thedecomposition of the resin expansion molded body without the oxidationof aluminum. The heating temperature can be appropriately determined inaccordance with the type of the resin expansion molded body. The heatingtemperature must be lower than the melting point (660° C.) of aluminumso as not to melt aluminum. A preferred temperature range is 500° C. ormore and 600° C. or less. A negative potential to be applied is on theminus side of the reduction potential of aluminum and on the plus sideof the reduction potential of the cation in a molten salt.

The molten salt used in the decomposition of a resin by heating may bean alkaline metal or alkaline earth metal halide salt such that thealuminum electrode potential is less-noble. More specifically, apreferred molten salt contains one or more selected from the groupconsisting of lithium chloride (LiCl), potassium chloride (KCl), sodiumchloride (NaCl), and aluminum chloride (AlCl₃). Removal of the resin bysuch a method can result in porous aluminum having a thin oxide layer ona surface thereof (a low oxygen content) and a low carbon content.

FIG. 4 is a schematic view of a cross-section taken along the line A-A′in FIG. 2( d). An aluminum layer composed of the electrically conductivelayer 2 and the aluminum plated layer 3 has a tubular skeletonstructure. A cavity 4 in the skeleton structure has almost triangularsections. The thickness (t1) of the aluminum layer at the vertexes ofeach of the triangular sections is greater than the thickness (t2) ofthe aluminum layer at the middle of each side of the triangularsections. This is probably because an electric field is concentrated atthe corners (the vertexes of a triangular section) in the formation ofthe aluminum layer by plating. Thus, in an aluminum structuremanufactured by a method according to the present invention, theskeleton structure has almost triangular sections, 2 0 and the aluminumlayer has a larger thickness at the vertexes of each of the triangularsections than at the middle of each of the triangular sections.

(Lithium-Ion Battery)

A battery electrode material and a battery each including porousaluminum will be described below. When porous aluminum is used in apositive electrode of a lithium-ion battery, the active material may belithium cobalt oxide (LiCoO₂), lithium manganese oxide (LiMn₂O₄), orlithium nickel dioxide (LiNiO₂). The active material is used incombination with a conduction aid and a binder. In a known positiveelectrode material for lithium-ion batteries, an active material isapplied to the surface of aluminum foil. In order to increase thebattery capacity per unit area, the application thickness of the activematerial is increased. In order to effectively utilize the activematerial, the active material must be in electrical contact with thealuminum foil. Thus, the active material is mixed with a conduction aid.Porous aluminum according to the present invention has a high porosityand a large surface area per unit area. Thus, even a thin layer of theactive material on the surface of the porous aluminum can effectivelyutilize the active material, increasing the battery capacity anddecreasing the amount of conduction aid to be mixed with. Lithium-ionbatteries include the positive electrode material described above as thepositive electrode, graphite as the negative electrode, and an organicelectrolyte as the electrolyte. Such lithium-ion batteries can have anincreased capacity even with a small electrode area and accordingly havea higher energy density than conventional lithium-ion batteries.

(Molten Salt Battery)

The porous aluminum can also be used as an electrode material for moltensalt batteries. When the porous aluminum is used as a positive electrodematerial, the active material is a metal compound, such as sodiumchromite (NaCrO₂) or titanium disulfide (TiS₂), into which a cation of amolten salt serving as an electrolyte can be intercalated. The activematerial is used in combination with a conduction aid and a binder. Theconduction aid may be acetylene black. The binder may bepolytetrafluoroethylene (PTFE). For the active material of sodiumchromate and the conduction aid of acetylene black, the binder ispreferably PTFE because PTFE can tightly bind sodium chromate andacetylene black.

The porous aluminum can also be used as a negative electrode materialfor molten salt batteries. When the porous aluminum is used as anegative electrode material, the active material may be sodium alone, analloy of sodium and another metal, or carbon. Sodium has a melting pointof approximately 98° C. and becomes softer with an increase intemperature. Thus, it is preferable to alloy sodium with another metal(such as Si, Sn, or In). In particular, an alloy of sodium and Sn ispreferred because of its excellent handleability. Sodium or a sodiumalloy can be supported on the surface of the porous aluminum byelectroplating, hot dipping, or another method. Alternatively, a metal(such as Si) to be alloyed with sodium may be deposited on the porousaluminum by plating and converted into a sodium alloy by charging themolten salt battery.

FIG. 6 is a schematic cross-sectional view of a molten salt batterymanufactured by using the battery electrode material described above.The molten salt battery includes a positive electrode 121, in which apositive electrode active material is supported on the surface of thealuminum skeleton of porous aluminum, a negative electrode 122, in whicha negative electrode active material is supported on the surface of thealuminum skeleton of porous aluminum, and a separator 123 impregnatedwith a molten salt electrolyte, in a case 127. A pressing member 126 isdisposed between the top surface of the case 127 and the negativeelectrode. The pressing member 126 includes a presser plate 124 and aspring 125 for pressing the presser plate. The pressing member canuniformly press the positive electrode 121, the negative electrode 122,and the separator 123 into contact with one another even when thevolumes of them have changed. A collector (porous aluminum) of thepositive electrode 121 and a collector (porous aluminum) of the negativeelectrode 122 are connected to a positive electrode terminal 128 and anegative electrode terminal 129, respectively, through a lead wire 130.

The molten salt serving as an electrolyte may be an inorganic salt or anorganic salt that can melt at the operating temperature. The cation ofthe molten salt may be one or more selected from alkaline metals, suchas lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium(Cs), and alkaline earth metals, such as beryllium (Be), magnesium (Mg),calcium (Ca), strontium (Sr), and barium (Ba).

In order to decrease the melting point of the molten salt, it ispreferable to use a mixture of at least two salts. For example, use ofpotassium bis(fluorosulfonyl)amide (KFSA) and sodiumbis(fluorosulfonyl)amide (NaFSA) in combination can decrease the batteryoperating temperature to 90° C. or less.

The molten salt is used in the form of a separator impregnated with themolten salt. The separator prevents the contact between the positiveelectrode and the negative electrode and may be a glass nonwoven fabricor porous resin. A laminate of the positive electrode, the negativeelectrode, and the separator impregnated with the molten salt housed ina case is used as a battery.

(Electrical Double Layer Capacitor)

The porous aluminum can also be used as an electrode material forelectrical double layer capacitors. When the porous aluminum is used asan electrode material for an electrical double layer capacitor, theelectrode active material may be activated carbon. The activated carbonis used in combination with a conduction aid and a binder. Theconduction aid may be graphite or carbon nano-tube. The binder may bepolytetrafluoroethylene (PTFE) or styrene-butadiene rubber.

FIG. 7 is a schematic cross-sectional view of an electrical double layercapacitor manufactured by using the electrode material for an electricaldouble layer capacitor. A polarizable electrode 141 is disposed in anorganic electrolyte 143 partitioned with a separator 142. Thepolarizable electrode 141 is made of an electrode material, which is anelectrode active material supported on the porous aluminum. Theelectrode material 141 is connected to a lead wire 144. All thecomponents are housed in a case 145. Use of the porous aluminum as acollector can increase the surface area of the collector. Thus, even athin layer of activated carbon as the active material on the surface ofthe porous aluminum can result in an electrical double layer capacitorwith a high power and a high capacity.

Although the resin expansion molded body is used as the resin moldedbody as described above, the present invention is not limited to theresin expansion molded body. A resin molded body having any shape can beused to manufacture an aluminum structure having a desired shape.

(Example: Manufacture of Porous Aluminum: Formation of Aluminum Layer byVapor Deposition Method)

An example of the manufacture of porous aluminum will be specificallydescribed below. A urethane foam having a thickness of 1 mm, a porosityof 95%, and approximately 20 pores per centimeter was prepared as aresin expansion molded body and was cut into a 10 mm×30 m square. Vapordeposition of aluminum on the surface of the urethane foam was performedto form an electrically conductive layer having a thickness ofapproximately 0.3 μm.

(Anode Electrolysis)

The urethane foam having an electrically conductive layer on the surfacethereof was mounted in a jig having an electricity supply function andwas then dipped in a molten salt aluminum plating bath (67% by moleAlCl₃-33% by mole EMIC) at a temperature of 40° C. The jig holding theurethane foam was connected to the anode of a rectifier, and an aluminumplate (purity 99.99%) of the counter electrode was connected to thecathode. A direct current having a current density of 1 A/dm² wasapplied for one minute to perform anode electrolysis. The calculation ofthe current density was based on the apparent area of the porousaluminum.

(Molten Salt Plating)

While the urethane foam having an electrically conductive layer on thesurface thereof was dipped in the molten salt aluminum plating bath, theanode and the cathode of the rectifier was switched therebetween. Adirect current was then applied to the urethane foam at a currentdensity of 3.6 A/dm² at a temperature of 40° C. for 90 minutes toperform aluminum plating.

(Manufacture of Porous Aluminum: Decomposition of Resin Expansion MoldedBody)

The resin foam having the aluminum plated layer was dipped in a LiCl-KCleutectic molten salt at a temperature of 500° C. A negative potential of−1 V was applied to the resin foam for 30 minutes. Air bubbles weregenerated in the molten salt, indicating the decomposition reaction ofthe polyurethane. The product was cooled to room temperature in theatmosphere and was washed with water to remove the molten salt, thusforming porous aluminum. The amount of aluminum deposit was 150 g/m².FIG. 8 is a scanning electron microscope (SEM) photograph of the porousaluminum.

The porous aluminum was dissolved in nitromuriatic acid and wassubjected to an inductively-coupled plasma emission spectrometer. Thealuminum purity was 99.1% by mass. The carbon content was 0.8% by massas measured by an infrared absorption method after combustion in ahigh-frequency induction furnace in accordance with Japan IndustrialStandard G1211. The energy dispersive X-ray spectroscopy (EDX) of thesurface at an accelerating voltage of 15 kV showed a negligible oxygenpeak, indicating that the oxygen content of the porous aluminum waslower than the detection limit of EDX (3.1% by mass).

(Evaluation of Porous Aluminum in Battery)

The practical evaluation of porous aluminum used as a battery electrodewill be described below in comparison with a conventional structurehaving an aluminum foil electrode.

A positive electrode active material LiCoO₂ having an average particlediameter of 7 μm, a conduction aid carbon black, and a binder resinpolyvinylidene fluoride were mixed at 10:1:1 (mass ratio). A solventN-methyl-2-pyrrolidone was added to the mixture to prepare a paste.Porous aluminum having a three-dimensional network structure and aporosity of approximately 95% was filled with the paste, was dried undervacuum at 150° C., and was role-pressed to a thickness corresponding to70% of the initial thickness to form a battery electrode material(positive electrode). The battery electrode material was punched in adiameter of 10 mm and was fixed to a coin battery container made ofstainless steel SUS304 by spot welding. The positive electrode fillingcapacity was 2.4 mAh.

For comparison purposes, the mixture paste of LiCoO₂, carbon black, andpolyvinylidene fluoride was applied to aluminum foil having a thicknessof 20 μm and was dried and role-pressed in the same manner as describedabove to prepare a battery electrode material (positive electrode). Thebattery electrode material was punched in a diameter of 10 mm and wasfixed to a coin battery container made of stainless steel SUS304 by spotwelding. The positive electrode filling capacity was 0.24 mAh.

A polypropylene porous film having a thickness of 25 μm was used as aseparator. A solution of 1 M LiPF₆ in ethylene carbonate (EC)/diethylcarbonate (DEC) (volume ratio 1:1) was dropped at 0.1 ml/cm² on theseparator, which was then subjected to vacuum impregnation. A lithiumaluminum foil having a thickness of 20 μm and a diameter of 11 mm wasfixed to the top lid of a coin battery container as a negativeelectrode. The battery electrode material (positive electrode), theseparator, and the negative electrode were laminated in this order andwere caulked with a Viton (registered trademark) o-ring placed betweenthe top lid and the bottom lid to manufacture a battery. In deepdischarge, the upper limit voltage was 4.2 V, and the lower limitvoltage was 3.0 V. Charging to the positive electrode filling capacitywas followed by discharging at each discharge rate. The lithiumsecondary battery containing the porous aluminum as the positiveelectrode material had a capacity approximately five times the capacityof a conventional battery containing aluminum foil as the electrodematerial at 0.2 C.

The above description includes the following characteristics.

(Additional Entry 1)

A manufacturing method of an aluminum structure, including a conductivetreatment process of forming an electrically conductive layer made ofaluminum on a surface of a resin molded body and a plating process ofplating the resin molded body subjected to the conductive treatmentprocess with aluminum in a first molten salt bath, wherein while theresin molded body having the aluminum plated layer is dipped in a secondmolten salt and while a negative potential is applied to the aluminumplated layer, the resin molded body is heated to a temperature of themelting point of aluminum or less to decompose the resin molded body.

Additional Entry 2

The manufacturing method of porous aluminum according to AdditionalEntry 1, wherein the resin molded body is a resin expansion molded bodyhaving contiguous pores.

Additional Entry 3

An electrode material in which an active material is supported on analuminum surface of an aluminum structure according to the presentinvention.

Additional Entry 4

A battery containing the electrode material according to AdditionalEntry 3 in one or both of the positive electrode and the negativeelectrode.

Additional Entry 5

An electrical double layer capacitor containing the electrode materialaccording to Additional Entry 3 as an electrode.

Additional Entry 6

A filtration filter including an aluminum structure according to thepresent invention.

Additional Entry 7

A catalyst support in which a catalyst is supported on the surface of analuminum structure according to the present invention.

Industrial Applicability

The present invention can provide a structure in which a surface of aresin molded body is plated with aluminum and an aluminum structuremanufactured by removing the resin molded body from the structure. Thus,the present invention can be widely applied as porous aluminum to caseswhere the characteristics of aluminum can be exploited, for example, inelectric materials, such as battery electrodes, various filters forfiltration, and catalyst supports.

Reference Signs List

1 Resin Foam

2 Electrically Conductive Layer

3 Aluminum Plated Layer

4 Cavity

21 a, 21 b Plating Bath

22 Strip of Resin

23, 28 Plating Bath

24 Cylindrical Electrode

25, 27 Positive Electrode

26 Electrode Roller

121 Positive Electrode

122 Negative Electrode

123 Separator

124 Presser Plate

125 Spring

126 Pressing Member

127 Case

128 Positive Electrode Terminal

129 Negative Electrode Terminal

130 Lead Wire

141 Polarizable Electrode

142 Separator

143 Organic Electrolyte

144 Lead Wire

145 Case

1. A manufacturing method of an aluminum structure, comprising: aconductive treatment process of forming an electrically conductive layermade of aluminum on a surface of a resin molded body; and a platingprocess of plating the resin molded body subjected to the conductivetreatment process with aluminum in a molten salt bath.
 2. Themanufacturing method of an aluminum structure according to claim 1,further comprising an anode electrolysis process of performingelectrolysis treatment using the electrically conductive layer as ananode between the conductive treatment process and the plating process.3. The manufacturing method of an aluminum structure according to claim1, wherein the resin molded body subjected to the conductive treatmentprocess is transported between the conductive treatment process and theplating process without being exposed to an oxidizing atmosphere.
 4. Themanufacturing method of an aluminum structure according to claim 1,wherein the conductive treatment process is a process of depositingaluminum on the surface of the resin molded body by a gas phase method.5. The manufacturing method of an aluminum structure according to claim1, wherein the conductive treatment process is a process of dipping theresin molded body in a coating material containing aluminum to depositaluminum on the surface of the resin molded body.
 6. The manufacturingmethod of an aluminum structure according to claim 1, wherein the resinmolded body is a porous resin article having a three-dimensional networkstructure.
 7. The manufacturing method of an aluminum structureaccording to claim 1, wherein the resin molded body is made of urethaneor melamine.
 8. The manufacturing method of an aluminum structureaccording to claim 1, further comprising a process of removing the resinmolded body after the plating process.
 9. An aluminum structuremanufactured by the method according to claim
 1. 10. An aluminumstructure, comprising an aluminum layer having a thickness in the rangeof 1 to 100 μm as a metal layer, wherein the metal layer has an aluminumpurity of 99.0% or more and a carbon content of 1.0% or less andcontains inevitable impurities as the balance.
 11. The aluminumstructure according to claim 10, further comprising a resin molded body,on which the metal layer is disposed.
 12. The aluminum structureaccording to claim 10, wherein the aluminum layer has a tubular skeletonstructure and forms a porous body having generally contiguous pores. 13.The aluminum structure according to claim 12, wherein the skeletonstructure has almost triangular sections, and the aluminum layer has alarger thickness at the vertexes of each of the triangular sections thanat the middle of each side of the triangular sections.